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Quantification of Fluorine Density in the Outermost Atomic Layer R. D. van de Grampel,†,§ W. Ming,*,† A. Gildenpfennig,‡ J. Laven,† H. H. Brongersma,‡ G. de With,† and R. van der Linde† Laboratory of Coatings Technology, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, and Calipso BV, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Received July 18, 2003 The outermost atomic layer of perfluorinated thiol monolayers on gold and poly(tetrafluoroethylene) (PTFE) is analyzed by low-energy ion scattering. Absolute quantification of fluorine density in this layer was achieved after calibrating the fluorine signal with a freshly cleaved LiF(100) single crystal. The fluorine density of monolayers of a C8F17-thiol on gold was 1.48 × 1015 F atoms/cm2, whereas for PTFE a value of 1.24 × 1015 F atoms/cm2 was observed. This difference was explained by the different tilt angles of the thiol on gold and PTFE chains with respect to the surface normal. Both a configurational and a molecular interpretation on the perfluorinated thiol monolayer on gold are given.
1. Introduction 1.1. General. The surface of materials often plays a central role for their applications. Understanding of the physical and chemical properties of surfaces is of paramount importance for the design of novel materials. Fluorinated polymers belong to a class of materials with exceptional properties. These properties originate from the nature of the carbon-fluorine bond and the orientation of the fluorinated tail. From both a fundamental and an application point of view, it is desirable to gain control over the wetting properties of fluorinated materials. Modern surface analytical methods such as X-ray photoelectron spectroscopy (XPS),1 time-of-flight secondary ion mass spectroscopy (TOF-SIMS),2 atomic force microscopy (AFM),3 scanning force microscopy (SFM),4 and nearedge X-ray absorption fine structure (NEXAFS)5 have been used to elucidate many aspects of the chemical composition as well as the structure of fluorinated surfaces. These methods provide useful information on the spatial variation in chemical composition near the surfaces. But none of them can exclusively analyze the composition of the outermost atomic layer. * To whom correspondence should be addressed. Fax: +31-402445619. E-mail:
[email protected]. † Eindhoven University of Technology. ‡ Calipso BV. § Current address: General Electric Plastics, P.O. Box 117, 4600 ACBergenopZoom,TheNetherlands.E-mail: robert.vandegrampel@ gepex.ge.com. (1) (a) Zhang, H.; Marra, K. G.; Ho, T.; Chapman, T.; Gardella, J. A., Jr. Macromolecules 1996, 29, 1660. (b) Tsibouklis, J.; Stone, M.; Thorpe, A. A.; Graham, P.; Nevell, T. G.; Ewen, R. J. Langmuir, 1999, 15, 7076. (2) Bo¨ker, A.; Reihs, K.; Wang, J.; Stadler, R.; Ober, C. K. Macromolecules 2000, 33, 1310. (3) (a) Sauer, B. B.; McLean, R. S.; Thomas, R. R. Langmuir 1998, 14, 3045. (b) Alves, C. A.; Porter, M. D. Langmuir 1993, 9, 3507. (c) Jaschke, M.; Scho¨nherr, H.; Wolf, H.; Butt, H.-J.; Bamberg, E.; Besocke, M. K.; Ringsdorf, H. J. Phys. Chem. 1996, 100, 2290. (4) (a) Sheiko, S. S. Adv. Polym. Sci. 1999, 151, 61. (b) Scho¨nherr, H.; Vancso, G. J. Polymer 1998, 39, 5705. (c) Gesquie`re, A.; AbdelMottaleb, M. M.; De Schryver, F. C. Langmuir 1999, 15, 6821. (5) (a) Castner, D. G.; Lewis, K. B., Jr.; Fischer, D. A.; Ratner, B. D.; Gland, J. L. Langmuir 1993, 9, 537. (b) Genzer, J.; Sivaniah, E.; Kramer, E. J.; Wang, J.; Ko¨rner, H.; Xiang, M.; Char, K.; Ober, C. K.; DeKoven, B. M.; Bubeck, R. A.; Chaudhury, M. K.; Sambasivan, S.; Fischer, D. A. Macromolecules 2000, 33, 1882.
Nevertheless, from the nature of the forces involved, for example, van der Waals interactions, it is expected that the outermost atomic layer of the surface predominantly determines the wettability. Consequently, the wettability of an organic material is not related to the overall chemical composition but rather depends on the chemical nature of the outermost functional groups. Zisman6 observed that the surface free energy decreases depending on the chemical composition of the material in the sequence -CH2- > -CH3 > -CF2- > -CF3. Therefore, knowledge of the interfacial behavior (e.g., surface energies) in relation to surface composition will contribute to a better understanding and prediction of adhesion and wetting phenomena. This means that characterization of the chemical composition of the outermost atomic layer is highly relevant. Low-energy ion scattering (LEIS), also referred to as ion scattering spectroscopy (ISS), is a unique technique that exclusively probes the first atomic layer of the surface (even when the sample surface is not smooth).7 1.2. LEIS. In a LEIS experiment, the sample is bombarded with low-energy noble gas ions. The energy of the ions that are elastically backscattered provides direct information on the masses of the atoms from which the ions are scattered. Such an ion scattering experiment is similar to a “billiard ball” collision; that is, energy and momentum are conserved in a collision. LEIS is a surface technique sensitive to the outermost atomic layer only.8 This is due to the fact that primary ions that collide with sample atoms have a high probability to capture an electron leading to neutralization. For ions that penetrate beyond the first atomic layer, the interaction time is so long that all ions will be effectively neutralized. Since electrostatic deflection is used in the analyzer, neutral species are not detected. This results in a negligible scattered-ion yield from atoms below the first atomic layer. (6) Zisman, W. A. In Contact Angle, Wettability and Adhesion; Fowkes, F. W., Ed.; ACS Symposium Series, Vol. 43; American Chemical Society: Washington, DC, 1964. (7) Van den Oetelaar, L. C. A.; Nooij, O. W.; Oerlemans, S.; Denier van der Gon, A. W.; Brongersma, H. H.; Lefferts, L.; Roosenband, A. G.; van Veen, J. A. R. J. Phys. Chem. B 1998, 102, 3445. (8) Brongersma, H. H.; Mul, P. M. Chem. Phys. Lett. 1972, 14, 380.
10.1021/la0353071 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/18/2003
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In a LEIS experiment, the ion scattering yield, Si, of an ion scattered from element i is given by
Si ∝ Ici(dσi/dΩ)Pi+ where I is the incident ion current, ci is the effective atomic surface density of element i, (dσi/dΩ) is the differential cross section for scattering from element i, and Pi+ is the ion fraction after scattering from element i. The neutralization of the ions depends on the interaction time. The ion fraction is generally given by
Pi+ ) exp(-vc/v) where v denotes the velocity of the ion and vc is a characteristic neutralization constant. Since the ion neutralization involves the electron transfer from the surface to the ion, one would expect that the precise electronic structure determines the value of Pi+. However, this is generally not the case. From energy (velocity) dependent experiments it is found, for instance, that PAl+ is the same for scattering from Al in Al or Al2O3.9 Likewise, no difference for the value of PSi+ was found for scattering from Si in Si or SiO2.10 Thus, in these cases there is no matrix effect that distorts the interpretation of scattered intensities, in contrast to, for example, the case of static SIMS analysis. More recently, similar observations of the absence of a matrix effect have been made also for organic materials.11 The sensitivity of the LEIS technique differs from element to element. It depends on the mass and nuclear charge of the projectile ion, the mass of the target atom, and the energy levels of, in particular, the inner-shell electrons of the target atom in relation to the levels of the ion. With a given probing ion, the sensitivity for most elements in the periodic table of elements does not differ by more than a factor of 10. Results as published in the literature12 usually are hampered by the lack of an absolute calibration method. The reason is that it is very difficult to determine P+ accurately. When an absolute calibration is necessary, reference samples with a well-defined composition in the outermost atomic layer are needed. In this paper, we will present an absolute calibration method for determining the fluorine density in the first atomic layer of materials, using a LiF single crystal as a calibration standard. Screening effects such as shadowing and shielding13,14 additionally led to a molecular interpretation of the monolayers of C8F17-thiol (F8-thiol) on gold. 2. Experimental Section 2.1. Materials. LiF(100) single crystals were purchased from ESCETE Single-Crystal Technology B.V. (Enschede, The Netherlands). 1H,1H,2H,2H-Perfluorodecanethiol (F8-thiol) and (9) Jacobs, J.-P.; Reijne, S.; Elfrink, R. J. M.; Mikhailov, S. N.; Brongersma, H. H. J. Vac. Sci. Technol., A 1994, 12, 2308. (10) Van Leerdam, G. C.; Brongersma, H. H. Surf. Sci. 1991, 254, 153. (11) Reijme, M.; Brongersma H. H. To be published. (12) (a) Mikhailov, S. N.; Elfrink, R. J. M.; Lacobs, J.-P.; van den Oetelaar, L. C. A.; Scanlon, P. J.; Bongersma, H. H. Nucl. Instr. Methods 1994, B39, 149. (b) Brongersma, H. H.; Carrere-Fontaine, M.; Cortenraad, R.; Denier van der Gon, A. W.; Scanlon, P. J.; Spolveri, I.; Cortigiani, B.; Bardi, U.; Taglauer, E.; Reiter, S.; Labich, S.; Bertrand, P.; Houssiau, L.; Speller, S.; Parascandola, S.; U ¨ nlu¨-Lachnitt, H.; Heiland, W. Nucl. Instr. Methods 1998, B142, 377. (13) Vargo, T. G.; Gardella, J. A., Jr.; Schmitt, R. L.; Hook, K. J.; Salvati, L., Jr. In Surface Characterization of Advanced Polymers, 1st ed.; Sabbatini, L., Zambonin, P. G., Eds.; VCH Verlagsgesellschaft: New York, 1993; Chapter 4. (14) Bertrand, P.; de Puydt, Y. Nucl. Instrum. Methods 1993, B78, 181.
van de Grampel et al. 1H,1H,2H,2H-perfluorooctanethiol (F6-thiol) were purchased from ABCR (Germany) and used without further purification. Additive-free poly(tetrafluoroethylene) (PTFE) film was purchased from Goodfellow and used as received. Deionized water and hexadecane (99+% anhydrous, Aldrich) were used as wetting liquids for the contact angle measurements. 2.2. Sample Preparation. Gold substrates were prepared by evaporating a gold layer of about 2000 Å thickness on a cleaned silicon wafer covered with a chromate layer of about 100 Å thickness. Monolayers of the thiols were deposited by soaking the freshly prepared gold substrate in a 1 mM thiol solution in absolute ethanol for a period of 3 h. Subsequently, the substrates were rinsed thoroughly with absolute ethanol and then spun dry on a spin coater at 5000 rpm for 60 s to obtain a surface free of voids. 2.3. LEIS Measurements. The LEIS measurements were performed with the Calipso setup,15 which is an improved version of the ERISS setup. The base pressure in the Calipso is 2 × 10-10 mbar. All LEIS measurements were carried out using 3 keV 3He+ ions. In this way, a high sensitivity to light elements is obtained. The kinetic energy of the ions backscattered over 145° is analyzed by a double toroidal analyzer and a position-sensitive detector. A shower of low-energy electrons during the LEIS measurements prevents charging of nonconducting samples. To avoid cross-contamination, only one sample at a time was present in the LEIS UHV analysis chamber. All experiments were carried out under static conditions. This means that the ion and the electron doses were kept sufficiently low to avoid damage of the sample. To obtain these static conditions, the ion beam was rastered over a sample area of 1.5 mm × 1.5 mm. The total ion dose was kept below 3 × 1013 3He+/cm2 (typical condition: beam current, 0.5 nA; measuring time,
